The present invention relates to the field of television signal broadcasting via over-the-air waves or through a cable network. More particularly, the present invention relates to an upconverter for processing a television signal prior to broadcast where the upconverter is configured to process either analog or digital television signals.
Television signals, comprising an audio signal component and a video signal component, are broadcast over-the-air, via satellite or through cable networks. These signals are then received by television sets which display the signals as intelligible television programming.
When television signals are originally generated, they are typically generated as intermediate frequency (IF) signals with a frequency around 45 MHz in the electromagnetic spectrum. In order for television signals to be broadcast, either over-the-air or through a cable network, and used by standard television sets, the television signals must be converted to higher frequency signals in the very high frequency (VHF) band or the ultra-high frequency (UHF) band.
This conversion is accomplished by a device known as an upconverter. The upconverter is located at the headend station where the television signals originate. The upconverter takes the IF television signal, converts it to a VHF or UHF signal, and provides the converted signal for broadcast. At a television signal headend, an upconverter or a separate upconversion circuit is used for each channel being broadcast by the service provider.
An example of an upconverter is illustrated in FIG. 1. The upconverter (100) receives an input IF television signal (101) which is provided to the conversion circuit (104). The conversion circuit (104) performs the upconversion. The resulting VHF or UHF signal (102) is output by the upconverter (100).
Traditionally, television signals have been analog signals. An analog television signal is based on a carrier wave (CW) at a specific frequency. Changes in the amplitude of the carrier wave, called modulations, are then made to convey the information of the television program encoded in the signal. Television sets respond to these modulations to generate the display of the television program being broadcast.
More recently, a trend has developed-to replace analog television signals with digital television signals. Digital signals can more quickly carry a greater volume of information. Consequently, the quality of the resulting picture and audio provided on a digital television set is significantly improved.
The problem with the shift to a digital format for television signal broadcasting is the need to replace existing analog broadcasting equipment and analog television sets with digital equipment. Consequently, there is a need in the art for an upconverter which can readily process both digital and analog television signals prior to broadcast. Such a capability eases the transition from an analog to a digital signal format.
One particular problem with providing such a dualmode upconverter is the difference in the carrier frequency band between the analog and digital broadcasting formats. For example, in the analog format, television signals are carried by amplitude modulating a base carrier wave. In North America the base analog carrier wave has a frequency of 45.75 MHz.
An upconverter operating in the analog format is shown in FIG. 1. This upconverter (100) includes an oscillator (103) that outputs a reference signal of 45.75 MHz. When a number of upconverters are being operated simultaneously to process various television channel signals being broadcast in the analog format, the phase of the various analog channel signals is preferably synchronized. This is known as Harmonically Related Carrier Operation (xe2x80x9cHRCxe2x80x9d).
In order to synchronize the phase of the channel signals, each upconverter (100) includes an identical oscillator (103) that may be synchronized to an external master reference. The oscillator (103) outputs the reference signal (105) with a frequency of 45.75 MHz to the modulator (106) that is generating the input IF television signal (101). The upconverters for all the channels being broadcast are typically interconnected by a control bus so as to coordinate their operation. Consequently, the phase the output RF television signals from the various upconverters can be coordinated.
Unfortunately, this reference frequency required for HRC in the analog format (45.75 MHz) has no special meaning in the digital format. Digital signals are not merely amplitude modulated, as analog signals are, but use modulation systems such as quadrature amplitude modulation (xe2x80x9cQAMxe2x80x9d) in which both amplitude and phase are modulated. In such a system, the analog reference frequency (45.75 MHz) is useless as a reference frequency for the modulator 106. Consequently, if the upconverter (100) is designed to process both analog and digital television signals, the output of the oscillator (103) will be required for HRC operation in the analog format, but will be useless or bothersome in the digital format.
Consequently, there is a need in the art for an upconverter and method of making and using the same which more readily allows the upconverter to properly process analog or digital television signals.
It is an object of the present invention to meet the above-described needs and others. Specifically, it is an object of the present invention to provide an upconverter and a method of making and using the same which can process digital or analog television signals and output an useful reference signal in either format.
Additional objects, advantages and novel features of the invention will be set forth in the description which follows or may be learned by those skilled in the art through reading these materials or practicing the invention. The objects and advantages of the invention may be achieved through the means recited in the attached claims.
To achieve these stated and other objects, the present invention may be embodied and described as an upconverter for converting an IF television signal to a VHF or UHF signal that processes either digital or analog television signals with equal effectiveness. The upconverter of the present invention may include an upconversion circuit for receiving an input television signal and outputting a converted television signal; and a reference signal circuit for supplying a reference signal to a modulator external to the upconversion circuit. The reference signal circuit provides a first reference signal if the input television signal is an analog signal and provides a second reference signal if the input television signal is a digital signal. Preferably, the first reference signal has a frequency of 45.75 MHz and the second reference signal has a frequency of 44 MHz.
The reference signal circuit preferably includes a voltage controlled oscillator with an output range including both the first and second reference signals. A phase locked loop circuit is used for controlling the oscillator to output either the first reference signal or the second reference signal.
A control circuit may automatically determine whether the input television signal is a digital or analog signal and configure the phase locked loop circuit accordingly to produce either the first or second reference signal. The control circuit may sample the input television signal to determine whether the input television signal is analog or digital. Alternatively, the control circuit may receive a control signal indicating whether the input television signal is a digital or analog signal.
The present invention also encompasses a method of upconverting an IF television signal to a VHF or UHF signal including outputting a first reference signal if the input television signal is an analog signal and a second reference signal if the input television signal is a digital signal. The method may also include determining with a control circuit whether the input television signal is a digital or analog signal and selectively providing the first or second reference signal in response.
The method of the present invention preferably further includes the steps of selectively generating the first or the second reference signal with a voltage controlled oscillator, controlling the oscillator with a phase locked loop circuit, and configuring the phase locked loop circuit according to whether the input television signal is a digital or analog signal with a control circuit.